In the car I use an Icom IC-2820 as my VHF/UHF radio. This has a packet port on it and previously I used to use an APRS tracker on it which would be switched on and off with the car ignition via a voltage sense relay. Since getting a new car I have been unable to find a +12v or 0v ignition switched wire in the boot. It means I needed to run one myself from where I could fine one in the front, not so easy in a Nissan 350z, but instead I decided to mod the IC-2820 so that it would provide +12v on on the packet port when the radio was on.
Which PIN to use?
The IC-2820 uses the standard 6 pin mini-din packet connections. These are;
I needed to think which one I could sacrifice for the switched 12v. Most APRS is on 1200 baud (PIN5) so the 9600 pin (PIN4) could be used. APRS PIC based trackers have carrier detect (DCD) but normal this is based on audio line activity/level and does not use PIN6 SQL. The rest of the pins are essential for the APR modem to interface to the radio.
With these PIN4 or PIN6 in mind it was time to check the IC-2820 Schematic and PCB for which is best, I means easiest, to use. Here is the schematic of the components immediately inline with the packet port.
We need to stop 12v getting into the circuit of the PIN we end up using. Looking at the schematic; to isolate PIN4 we need to remove R594 and to isolate PIN6 we need to remove EP24 (filter). Lets look at the PCB to see where each of these components are to know how easy it will be to remove them;
PIN4 goes directly to the underside of the PCB on a via so is not really usable without removing the main PCB from the case to get access to R594. However EP24 is right next to PIN6 on the top side of the PCB so removing that to isolate PIN6 to carry +12v is the easiest way to go. So PIN6 SQL will carry the switch +12v on the packet port to the attached APRS modem.
Finding a Switched +12v
Most modern radios use the CPU, or dedicated circuit, to control the main power to the radio from a momentary power push button on the front panel. This works by supplying the CPU or dedicated circuit a low voltage permanently (also low current in this state) which allows it to monitor the power button state. When the power button is pressed then a control voltage is send back to section of the radio controlling the main DC power which enables either a relay or a high power transistor thus supplying the rest of the radio with 12v at a higher current capability.
The advantages of such a method is that only low current connections are needed to be be brought to the CPU/control circuit. On a radio with a remote head such as the IC-2820 this is the only practical way, otherwise you would need to bring high current DC cables to the head and fit a high current switch, this would increase the costs, add design challenges not to mention the potential for voltage drop. Also by using a CPU for power control then APO (Automatic Power Off) can be implemented. But I digress from the task in hand…
So for this mod I needed to find a location on the radio where the +12v is switched with power on/off. I didn’t want this to be the CPU control voltage as it will not have much current capability. So again back to the IC-2820 schematic and it’s easy to find the point just after Q81, tagged VCC, I can take a high current switched +12v DC connection from to connect to PIN6 on the packet port;
This VCC point after Q81 gives me the +12v switched voltage I needed. Next was to find a location on the PCB which was connected to VCC. Q81 is on mounted on the bottom side of the main PCB and would possibly require removing the PCB from the housing to get at Q81/VCC.
The schematic shows that VCC also goes to PIN9 on IC38 which is the audio amp IC.
Although IC38 is mounted on the bottom side of the main PCB it is a through hole device so the pins are accessible from the top side, result.
Connecting the 2 points together
Here is a picture (click to enlarge) of the bottom of the radio with the front of the radio to the left (note the schematic above is showing the bottom side so is the opposite way round to tis photo of the top side), and gives an overview of how the connection between IC38 PIN9 (Switched +12v point) and PIN6 on the packet port is done in practice, the white wire is the mod carrying the switched +12v from PIN9 of IC38 to PIN6 of the packet port.
This is the Switched +12v point on IC38 closer up (click to enlarge) with the wire connected to PIN9, PIN1 of IC38 is towards the front of the radio.
This is a PCB view of the packet port and shows PIN6 where PIN9 of IC38 connects to. It also shows the location of EP24 which needs to be removed, see text below.
Here are 2 images (click to enlarge) showing EP24 after I removed (**tombstoned) it and the wire carrying the Switched +12v from PIN9 on IC38 to PIN6.
**BTW by tombstoned I mean I reworked EP24 so only one pad is soldered so if I want to undo the mod at a later date EP24 is where it should be and only needs soldering back across both pads.
Testing after the mod I get the result I expected. When I switch on my IC-2820 the attached Mobilinkd Bluetooth TNC also powers on, when the radio is switched off so does the TNC.
I hope you find this useful.